Most data that is used in this research is extracted from the OAG (Official Airline Guide) database. Data from the OAG database11 contains the flight schedule of all scheduled flights in the third week of June during the period 1990-1999, and the third week of February and September during the period 2003-2005. The OAG database contains for example data for each flight route about the departure and destination airport, the airline company, the type of aircraft, the flight frequency, the number of seats, the flight duration, and the travelled distance.
A second important dataset is based on the Quickaid.com Airport Directory12, which contains the exact coordinates of most airports. With help of these coordinates data can be visualized and analysed in a GIS program.
Other data that has been used in this research is obtained from Eurostat, and contains regional data of the number of tourists, the GDP (Gross Domestic Product), and the population density. The used Eurostat data originates from 2002 and 2003, these years contained the most complete dataset at the moment of this research. Other data is obtained from Geodan and contains different geographical data, like for example region borders, which is used in GIS programs. Geodan data originates from 2003.